1. Field of the Invention
This invention relates to a supercritical refrigeration cycle system of a vapor-compression-type comprising a plurality of evaporators in which the refrigeration pressure on high pressure side increases to at least the critical pressure.
2. Description of the Related Art
A conventional refrigeration cycle system of this type is known to include a compressor for compressing a refrigerant, a radiator for cooling the refrigerant discharged from the compressor, a first decompressor and a second decompressor for reducing the pressure of the refrigerant flowing out of the radiator, a first evaporator for evaporating the refrigerant flowing out of the first decompressor, a second evaporator for evaporating the refrigerant flowing out of the second decompressor, and a solenoid valve for controlling the refrigerant flow from the radiator into the second decompressor, wherein the air blown into the front part of the compartment is cooled by the first evaporator and the air blown into the rear part of the compartments is cooled by the second evaporator (Japanese Unexamined Patent Publication No. 2000-35250 (Patent Document 1)).
As a measure for suppressing the production cost, on the other hand, a system in which the number of expansion valves for decompressing the refrigerant is reduced and the decompressed refrigerant is distributed to each evaporator has been proposed (Japanese Unexamined Patent Publication No. 2005-106318 (Patent Document 2)).
In the refrigeration cycle system described in Patent Document 1, however, if the low pressure of the refrigerant is reduced during the transient period of starting or increasing the rotational speed of the compressor, and because a temperature-type expansion valve is used as a second decompressor, the drop in the low pressure immediately acts to open the second decompressor as shown in the example of the behavior of starting the system using a mechanical expansion valve (see FIGS. 11A, 11B). Further, the temperature drop at the evaporator outlet is accompanied by the delay due to heat transmission and, therefore, the valve opening degree of the second decompressor is excessively increased temporarily, with the result that the refrigerant flow rate is not properly distributed to each evaporator, thereby posing the problem that the blowout air temperature, of the evaporator short in the refrigerant flow rate, increases.
In the case where an electrical expansion valve is used as a second decompressor, on the other hand, the low pressure has no effect. Even in the case where the low pressure drops during the transient period, therefore, the valve opening degree is not excessively increased. Although the detection of a superheat amount requires the detection of the refrigerant temperature at the outlet of the evaporator, an excessively fast response destabilizes the operation of the electrical expansion valve and leads to the problem of hunting, etc. To secure stability, the response to temperature detection is required to be somewhat slow. In the case where the thermal load or the rotational speed of the compressor undergo an abrupt change, therefore, the refrigerant flows excessively, temporarily, and the resultant increased superheat amount of the first evaporator may increase the blowout air temperature.
In the refrigeration cycle system described in Patent Document 2, on the other hand, the high-pressure refrigerant, after being decompressed in the expansion valve, is required to be sent to each evaporator by piping. In the automotive air conditioning system, for example, the refrigerant is sent to the front evaporator in the dashboard for the front seats on the one hand and must send the low-pressure low-temperature refrigerant to the rear evaporator for the rear seats through a long pipe. To suppress the heat loss in the long pipe and the frosting of the pipe, the pipe is required to be covered by a heat insulating material.